1. Field of the invention
This invention relates to an improved optically pumped laser. In particular, the optically pumped laser includes a laser diode array for generating optically pumped radiation having a uniform intensity distributed over a broad bandwidth and a lasant material with an absorption band for absorbing radiation within the above bandwidth.
2. Description of the Prior Art
A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms or molecules of an active medium or lasant material which have typically been excited from ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflective surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the lasant material, the spontaneous emission of a photon from an excited atom or molecule returning to its ground state can stimulate the emission of photons of identical energy from other excited atoms or molecules. As a consequence, the initial photon creates a cascade of photons between the mirrors of the optical cavity which are of identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity.
Excitation of the lasant material of a laser can be accomplished by a variety of methods, such as, by optical pumping, current injection or the use of an electrical discharge. Optical pumping involves the creation of a population inversion through the absorption of light by a lasant material. The use of light from noble gas arc lamps, tungsten-halogen lamps, light-emitting diodes, laser diodes and laser diode arrays to optically pump or excite the lasant material of a laser is well known.
In order to effect optical pumping, the photons delivered to the lasant material from a radiant source must be of a very precise character as within the absorption band of the lasant material. In particular, the pumping radiation must be of a wavelength which is absorbed by the lasant material to produce the required population inversion.
U.S. Pat. No. 3,624,545 issued to Ross describes an optically pumped solid state laser composed of a neodymium-doped yttrium aluminum garnet (Nd:YAG) rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S. Pat. No. 3,753,145 issued to Chesler, discloses the use of one or more light-emitting semiconductor diodes to end pump a Nd:YAG rod. The use of an array of pulsed laser diodes to end pump a solid lasant material such as neodymium-doped YAG is described in U.S. Pat. No. 3,982,201 issued to Rosenkrantz et al.
Lasers, such as semiconductor diode lasers, are activated by the application of an electrical current. Laser diodes are efficient pumps for optically pumped lasers since the output radiation from the laser pump is a single wavelength (or a very narrow band of wavelengths) which is selected in such a manner that matches the absorption band or peak of the lasant material to be optically pumped. Unfortunately, it is frequently difficult to match the output radiation of the laser pump with the appropriate absorption band of the lasant material, since the output radiation from the laser pump source is generally of a single wavelength which is selected in such a manner so as to precisely match the absorption band peak of the lasant material which is to be optically pumped. Moreover, temperature, pressure and aging of diode lasers significantly affect, alter and change the characteristics of laser diodes, by changing the wavelength of the output radiation of laser diode pumps. Thermoelectric heaters/coolers and sophisticated feedback circuits are utilized with a goal toward precisely matching the output radiation of laser diodes with the absorption band or peak of the lasant material. Over the years a number of laser diodes have been suggested for matching the output radiation of laser pumps with the absorption band of the lasant material, however, such laser diodes have resulted in varying degrees of success. It is therefore desirable to provide an improved optically pumped laser which overcomes most, if not all, of the above problems.
R. B. Allen, GaAlAs Diode Pumped Nd:YAG Laser, Technical Report AFAL-TR-72-319, Jan. 1973, pp. 1-9, describesthe results of a program to develop and test a laboratory model of a room-temperature GaAlAs diode-pumped Nd:YAG. The Report discloses a laser which produced a CW power of more than 80 mW in the TEMoo mode, which it was asserted, represented the highest level of TEMoo power reported to date for a diode-pumped laser. The GaAlAs light emitting diodes in this Report were selected for the best room temperature spectral match to the Nd:YAG absorption band near 805 nm. The CW operating characteristics of the 15 best of 17 individually fabricated subarrays are given in Table 1 (of the Allen report), which includes the peak emission wavelength at 25.degree. C. for 250 mA drive current. The distribution of the subarray is found in FIG. 4 (of the Allen report), which is a graph of the output power versus the peak emission wavelength. Twelve of the subarrays have peak emission wavelengths in the range of 805 nm plus or minus 5 nm. The other three have shorter wavelengths. The CW operating characteristics of the 2 worst of the 17 individually fabricated subarrays were ignored.
U.S. Pat. No. 3,946,331 issued to Pollack et al. describes a Nernst lamp for optical pumping of a solid state laser. The lamp materials were selected so that the light emitted was essentially concentrated in the relatively narrow pump region of the absorption spectrum of the laser crystal.
W. T. Tsang, Appl. Phys. Letter, Vol. 36, No. 6, 1980, pp. 441-443 discloses a multiwavelength transverse-junction-stripe laser, which is capable of emitting multiple predominantly single-longitudinal mode emissions at various wavelengths. In an example, four different outputs at 902.5, 879.3, 853.2 and 827.6 nm were obtained simultaneously from a single-wavelength TJS laser.
In contrast, none of the above references disclose or suggest an optically pumped laser comprising a laser diode array for generating optical pumping radiation, such pumping radiation having a bandwidth which is about 3 nm to about 15 nm wide and wherein the intensity of the pumping radiation is substantially uniformly distributed over such bandwidth, and a lasant material with an absorption band for receiving radiation within the bandwidth of the laser diode array.